Over the past several decades, UV-curing has given rise to new functional materials in various fields of polymer science. The attraction of photopolymerization stems from the excellent control over the spatial and temporal polymerization process,1,2 along with high cross-linking densities3,4 and low energy cost. To broaden the scope of potential applications for this technique, there is a need to introduce new polymers with unconventional functionality for the growing demand in industry. Polyelectrolytes have been filling a niche in polymer science as they have noticeable differences in their chemical and physical properties as compared to neutral polymers.5,6 This includes their conformation in solution, the propensity to form gels, and the potential to form layer-by-layer architectures on a surface.7-9 Bringing polyelectrolyte chemistry and photopolymerization together represents a paradigm shift in what may lead to new functional materials.
UV-curing is the process by which a thin layer or film of liquid monomer is polymerized using UV light. Polymerization propagates via free radical mechanisms using a light sensitive initiator and monomer containing, for example, acrylates, methacrylates, or styrenes. Radicals are formed by the cleavage of the initiator which commences polymerization. Reactions rates are initially extremely high as a large pool of unreacted monomer surrounds the radicals. As the polymer propagates, cross-linked networks are formed. This results in the vitrification of the polymer matrix during which propagation slows and the rate of termination increases. This is a diffusion-controlled process, as radicals no longer have the mobility required to continue the reaction. Typically within a second the reaction is complete and the material is a hard, cross-linked solid.
Thermal curing is currently the dominant method in the coatings industry. Despite its usefulness, considerable drawbacks still plague the thermal cure. Thermal curing generally requires use of a large amount of solvent to process and deposit the material, as the viscosity of the formulation is otherwise too high. After deposition of resin, ovens heat up the material to the desired temperature for up to an hour to effect the thermal curing step. This process is energy intensive and requires ventilation to handle resulting volatile organic compounds. Startup and shutdown times of these systems consume energy while not producing product. UV curing systems, on the other hand, display higher efficiency with few of the thermal processing issues.
The photopolymerization industry has yet to incorporate electrolytes into their coating formulations, despite potential uses for the resulting films. Polyelectrolyte films have mainly been employed as surface-modifying agents, exploiting their ionic properties.9 One use for charged polymers is for the fabrication of layer-by-layer (LbL) assemblies. Charged substrates are dipped back and forth between solutions of positively and negatively charged polymer. During each immersion, a thin layer of polyelectrolyte is adsorbed, and the surface charge is reversed. Depending on choice of solvent, the ionic strength of the solution, and polyelectrolyte, users are capable of controlling film thickness.9 Recently, LbL assemblies have been shown to exhibit excellent gas barrier properties, act as anti-corrosion coatings, and as vehicles for drug delivery.8,10,11 The broad scope of the technique stems from the use of charged molecules to form electrostatic bonds as opposed to covalent bonds, which require appropriate functionality to facilitate formation. LbL assemblies have also been made through various other means utilizing bio-recognition,12,13 hydrogen-bonding, or host-guest interactions.14,15 Currently, electrostatic assemblies dominate the field. To deposit the first charged polyelectrolyte, substrates are treated.10,11,16 Chemical treatments of glass, quartz, or metal are the most common method to create a charged surface. In cases where chemical treatment has not been an option, such as for wood or plastics, LbL deposition is not viable, limiting the scope of potential applications for the technique. Polyelectrolyte LbL assemblies have also been known to flow depending on temperature and humidity, which may result in decreased performance.10,11,17 A potential way to improve upon such problems would be to have the ability to form robust polyelectrolyte networks on any desired substrate.